Electronic Configuration
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge AS Level Physics 9702
Electron Shells Electrons in atoms are arranged in shells (also called energy levels) around the nucleus. Understanding how electrons are arranged is crucial for understanding chemical bonding and reactivity.
Electronic Configuration The arrangement of electrons in shells (energy levels) around the nucleus of an atom. Rules for Electron Arrangement 1. Maximum Number of Electrons per Shell Each electron shell can hold a maximum number of electrons: 1st shell (closest to nucleus): Maximum 2 electrons 2nd shell: Maximum 8 electrons 3rd shell: Maximum 8 electrons (for elements 1-20) 4th shell and beyond: More complex (beyond IGCSE scope for elements 1-20) Important For IGCSE, you only need to know the electronic configuration for the first 20 elements (hydrogen to calcium).
For these elements, remember: 2, 8, 8 pattern. 2. Filling Order Electrons fill the innermost shell first (lowest energy) Only when a shell is full do electrons occupy the next shell Shells fill in order: 1st → 2nd → 3rd → 4th The outermost shell is called the valence shell Writing Electronic Configuration Electronic configuration can be written in different ways: Method 1: Shell Notation (2.8.1 format) Write the number of electrons in each shell, separated by dots or commas.
Example: Sodium (11 electrons) = 2.8.1 (2 electrons in 1st shell, 8 in 2nd shell, 1 in 3rd shell) Method 2: Diagram Draw circles representing shells with crosses or dots representing electrons. Examples of Electronic Configurations Hydrogen (H) - 1 electron Electronic configuration: 1 1 electron in the 1st shell Helium (He) - 2 electrons Electronic configuration: 2 1st shell is full (2 electrons) Noble gas - very stable Carbon (C) - 6 electrons Electronic configuration: 2.4 2 electrons in 1st shell, 4 electrons in 2nd shell Oxygen (O) - 8 electrons Electronic configuration: 2.6 2 electrons in 1st shell, 6 electrons in 2nd shell Sodium (Na) - 11 electrons Electronic configuration: 2.8.1 2 in 1st shell, 8 in 2nd shell (full), 1 in 3rd shell Chlorine (Cl) - 17 electrons Electronic configuration: 2.8.7 2 in 1st shell, 8 in 2nd shell (full), 7 in 3rd shell Calcium (Ca) - 20 electrons Electronic configuration: 2.8.8.2 2 in 1st shell, 8 in 2nd shell, 8 in 3rd shell, 2 in 4th shell Electronic Configurations of First 20 Elements Element Symbol Atomic Number Electronic Configuration Hydrogen H 1 1 Helium He 2 2 Lithium Li 3 2.1 Beryllium Be 4 2.2 Boron B 5 2.3 Carbon C 6 2.4 Nitrogen N 7 2.5 Oxygen O 8 2.6 Fluorine F 9 2.7 Neon Ne 10 2.8 Sodium Na 11 2.8.1 Magnesium Mg 12 2.8.2 Aluminium Al 13 2.8.3 Silicon Si 14 2.8.4 Phosphorus P 15 2.8.5 Sulfur S 16 2.8.6 Chlorine Cl 17 2.8.7 Argon Ar 18 2.8.8 Potassium K 19 2.8.8.1 Calcium Ca 20 2.8.8.2 Valence Electrons Valence Electrons The electrons in the outermost shell of an atom.
These electrons are involved in chemical bonding. Valence electrons determine an element's chemical properties Elements in the same group have the same number of valence electrons Example: All Group 1 elements have 1 valence electron (Li: 2.1, Na: 2.8.1, K: 2.8.8.1) Atoms with full outer shells are very stable (noble gases) Key Concept The number of valence electrons determines how an element reacts.
Elements want to achieve a full outer shell (8 electrons, or 2 for the first shell) to become stable like noble gases. Noble Gas Configuration Noble gases (Group 0/18) have full outer shells, making them very stable and unreactive.
Helium (He): 2 - Full 1st shell Neon (Ne): 2.8 - Full 1st and 2nd shells Argon (Ar): 2.8.8 - Full 1st, 2nd, and 3rd shells Other elements gain, lose, or share electrons to achieve a noble gas configuration: Sodium (2.8.1) loses 1 electron → becomes Na⁺ (2.8) like neon Chlorine (2.8.7) gains 1 electron → becomes Cl⁻ (2.8.8) like argon Relationship to the Periodic Table Groups (Vertical Columns) Elements in the same group have the same number of valence electrons Group 1: 1 valence electron (Li: 2.1, Na: 2.8.1, K: 2.8.8.1) Group 7: 7 valence electrons (F: 2.7, Cl: 2.8.7) Group 0: Full outer shell (He: 2, Ne: 2.8, Ar: 2.8.8) Periods (Horizontal Rows) Period number = number of electron shells Period 1: 1 shell (H: 1, He: 2) Period 2: 2 shells (Li: 2.1 to Ne: 2.8) Period 3: 3 shells (Na: 2.8.1 to Ar: 2.8.8) Exam Tip You can work out the electronic configuration from the periodic table position: Period number tells you how many shells; Group number tells you how many valence electrons (for main groups).
How to Work Out Electronic Configuration Step-by-Step Method Find the atomic number - this is the number of electrons in a neutral atom Fill the 1st shell - maximum 2 electrons Fill the 2nd shell - maximum 8 electrons Fill the 3rd shell - maximum 8 electrons (for elements 1-20) Continue until all electrons are placed Example: Aluminum (Al) - Atomic number 13 13 electrons to arrange 1st shell: 2 electrons (11 remaining) 2nd shell: 8 electrons (3 remaining) 3rd shell: 3 electrons (0 remaining) Electronic configuration: 2.8.3 Electronic Configu…
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